The present invention relates to an integrated circuit package suitable for an integrated circuit used for processing digital signals at high speeds of several hundred Mb/s or more.
An integrated circuit package must fix hold a semiconductor chip and must protect the semiconductor chip from the environment. The integrated circuit (IC) package has the further function of supporting lead frames employed for supplying electric power and for receiving/transmitting various signals.
The integrated circuit package has still further functions of electrically insulating the semiconductor chip and discharging heat generated by the semiconductor chip.
With respect to silicon semiconductor integrated circuits, plastic packages are frequently employed. Plastic packages are made by sealing silicon semiconductor integrated circuit chips in plastic moldings. That is, after the lead frames and the electrode portions of the chip have been connected through wire-bonding to each other with metal wires, the entire structure is molded with a plastic material so as to be sealed.
Lead frames are usually provided on opposite longitudinal side surfaces of the package. A package having lead frames formed in lines on two opposite is called a DIP (dual-in-line package). Eight lead frames or more, sometimes 40 or more, are provided. The interval between the lead frames is standardized, and in many cases is selected to be about 2.5 to 2.9 mm. As the number of lead frames is increased, the package becomes large.
The plastic package has the advantage that its cost is low and an integrated circuit chip can be easily mounted thereon. Therefore, at present plastic packages accounts for about 80% of all produced integrated circuit packages.
The plastic package, however, is inferior in cooling properties as well as in its ability to hermetically seal. Therefore, the plastic package can be used only for an IC chip having a relatively small power dissipation. Because the chip can have only a small dissipation, the integration density or operational speed of the chip is limited.
For IC chips having a large power dissipation, it is required that the package not only have a higher thermal conductivity but also a thermal expansion coefficient approximate to that of the IC chip.
Accordingly, ceramic DIP integrated circuit packages made of alumina (Al.sub.2 O.sub.3) have been employed. These packages are constituted by two ceramic plates combined with each other.
Such a package is called a CERDIP because it has a DIP arrangement and is made of ceramics. At present, about 15% of all IC packages have the CERDIP arrangement.
The CERDIP package is produced as follows:
A number of lead frames are sealed with a low-melting-point glass on an elongated thin lower ceramic plate. A silicon semiconductor IC chip is die-mounted on the lower ceramic plate, and electrode portions of the IC chip are connected to the lead frames through wire-bonding with Al wires. An upper ceramic plate is fixed to the lower ceramic plate using sealing glass, and then a cover is attached.
The thus-arranged ceramic package has a usable temperature range wider than that of the plastic package. The ceramic package is therefore used for semiconductor devices which have high reliability requirements because the ceramic package has superior cooling properties as well as better hermeticity, and the thermal expansion coefficient of the ceramic package is approximately the same as the IC chip.
In a CERDIP, however, the electrode portions of the IC chip and the lead frames are directly connected to each other through wires, and therefore the lengths of the wires are unavoidably increased as the number of lead frames (pins) is increased.
Long wires may be tolerated in the case of an IC used for low-speed operations. However, the wires are required to be made as short as possible in the case of an IC used for high-speed operations.
Therefore, both the plastic DIP and the ceramic DIP (CERDIP) are both somewhat unsuitable for IC chips used for high-speed operations.
Moreover, silicon semiconductor ICs are themselves inherently satisfactory for high-speed operations.
In the case of a CMOS IC, generally, the maximum operating frequency is limited to about 50 MHz. CMOS ICs though have a small power dissipation, and therefore in many cases plastic packages can be used for CMOS ICs.
Further, for example, in the case of TTL devices, the maximum operating frequency is about 50 to 70 MHz.
In the case of an ECL (emitter-coupled logic) device which is a silicon digital IC capable of high-speed operation, generally, the maximum operating frequency is about 500 MHz. When an ECL device is operated at a high speed, however, the power dissipation is large.
The increase in power dissipation in ECL devices is caused by limitations in the material used in constructing such devices. Silicon semiconductor material is not suitable generally for high-speed operations because of its low electron mobility.
In order to achieve higher-speed operability, it is usually necessary to use an FET (MESFET, MODFET) made of a compound semiconductor such as GaAs or the like. This is because in materials the electron mobility is exceedingly high.
Development of GaAs integrated circuits has been occurring steadily. An integrated circuit mainly constituted by GaAs FETs or the like can generally operate at an ultra-high speed.
It is of course necessary not only or the IC chip itself to be capable of operating at high speeds, but also for its package to not prevent the high-speed operation of the chip.
In the case where a digital signal is processed at a speed of several hundreds Mb/s, generally the signal waveforms are rectangular. When a rectangular wave of a frequency F is expanded in a Fourier series, the series includes components higher in frequency than the fundamental frequency F. In order to perfectly reproduce the rectangular wave, the IC is required to operate at a higher frequency. For example, if a rectangular wave of a frequency of several hundred Mb/S is to be successfully handled, it is required that the IC device have a sinewave amplification capability of several GHz. Such a high-speed signal imposes strict conditions on the IC package.
A high-speed signal cannot be freely transmitted with a lead wire like a low-speed electric signal. The high-speed electric signal can be effectively transmitted only when a suitable relation is satisfied between the width of the lead wire, the thickness of the substrate, etc.
An electric conductor such as a lead wire or the like not only acts as a pure electric conductor but also has self-inductance L. When the frequency is high, the reactance L.omega. due to the self-induction L is not negligible. Further, a capacitance C is formed between the signal lines and the ground plane as well as between signal lines. Thus, in the case of a high-speed signal, the problem of L and C is always important. The characteristic impedance Z.sub.0 =.sqroot.L/C is determined by L and C per unit length. The characteristic impedance is required to be made constant along a transmission line because, if a discontinuity of the characteristic impedance is present a reflection is generated at the point of discontinuity. Further, it is necessary to terminate the end of the transmission line with an impedance equal to the characteristic impedance. If not, a signal energy reflection occurs at the end of the transmission line.
Conventionally, the foregoing problem is well known in design of a coaxial cable. In coaxial cables having a characteristic impedance of 50 .OMEGA., 75 .OMEGA., etc., the respective sizes of the signal line, the insulating layer, the ground sheath and the like are made constant so as to prevent the characteristic impedance from varying. Further, in the case of a high-speed signal, the coaxial cable is terminated with a resistor having the characteristic impedance.
The same conditions as described above are required also for IC packages used for high-speed ICs. In such devices, it is necessary that characteristic impedance be made constant and a termination resistor equal to the characteristic impedance provided.
Further, it is preferable to make the inductance L of the lead wire and the size of the wire as small as possible because, if L is large, it is difficult for the input signal reaching an input electrode of the IC to not be deteriorated in its waveform. Generally, the selfinduction of the electric conductor increases as the sectional area thereof becomes smaller or the wire is elongated.
In the package, there are present electric conductors such as lead frames, metallized wiring, an electric conductor pattern in the IC chip, bonding wires, etc. Among them, the electric conductor pattern and the wires are particularly thin.
The wires have a considerably large inductance L because they are thin, and therefore they should be made as short as possible. To this end, it is impossible to directly connect the electrode portions of the IC chip to the lead frames through wires, unlike the conventional IC package, because the wires become longer as the number of lead frame as is increased.
Further, there is a disadvantage in that the number of power sources is increased. TTL devices, CMOS devices, etc., presently on the market can be driven by a single power source. In the case of an IC having high-speed logical elements, however, three power sources are sometimes required. In such a case,.it is necessary to form four separate power source lines, including the ground line (of a reference potential).
Further, the power source and the ground line are required to be connected to various portions of the IC chip. Since it is preferable to make all wires as short as possible, metallized wirings are sometimes used for power source wirings and the ground wirings. For this purpose, ten metallized wirings or more are sometimes required to be used as the power source wires.
Further, in an IC used for high-speed operations, the heat generated from the semiconductor chip thereof is inevitably high. Accordingly, in a package used for high-speed ICs, it is particularly important to improve the cooling property thereof so as to increase the reliability of the device.
FIG. 1 shows an example of the best known conventional package used for high-speed integrated circuits. In the drawings, the package in the state where a ceramic cover plate is removed is illustrated. This package is called a ceramic package of the flat type so as to be distinguished from a CERDIP.
A square ceramic main plate 1 having a square opening at its central portion is provided at its lower surface with a bottom plate 11 fixed thereto. The bottom plate 11 is also made of ceramic. Metallized wiring 3 radially extending from the opening of the ceramic main plate 1 to side edges of the same are formed on the ceramic main plate 1. A ceramic frame 7 is fixed to the ceramic main plate 1. A cover plate (not shown) is attached to the frame 7.
Lead frames 10 are attached by brazing to the respective termination portions of the metallized wirings 3. Although the lead frames 10 extend in four directions in this example, the lead frames 10 may extend only in two directions so as to form a DIP arrangement. The metallized wirings include a signal line and power source line, which are disposed on the same plane. Including the cover plate, this package has four ceramic plates stacked one on the other.
The package constituted by three or more stacked ceramic plates as described above is called an MLCP (multi-laminated ceramic package).
At present, MLCPs account for about 3% of all IC packages. MLCPs, however, are exceedingly expensive in comparison with plastic packages.
When an IC chip is mounted on the package, the IC chip is die-mounted to the bottom plate 11, and electrode portions of the chip are connected to the start points of the metallized wirings by wire-bonding with gold wires.
In this package, the lead frames are connected to the wires through the metallized wiring, and hence it has an advantage that the wires are not long, even when the number of lead frames is increased.
When the number of input and output signals is increased to thereby increase the number of power source lines, however, the wires unavoidably become long. That is, since the intervals between the metallized wirings and the widths of each of the metallized wirings are limited, the opening portion of the ceramic plate must be several times as wide as the size of the IC chip if the number of wirings is increased. In such a case, the bonding wires become so long that the value of L become large, making it difficult to transmit signals to the semiconductor chip with no deterioration.
At present, the MLCP package of FIG. 1 is the most technically developed IC package. In this package, metallized wirings are formed on each of the ceramic plates, which are vertically stacked one on the other. Even if the number of wirings is increased, it is not necessary to make the opening portions of the respective ceramic plates longer, and therefore the wires connected the IC chip to the metallized wirings can be made shorter than was possible in the DIP-type packages. Accordingly, this package has the advantage that the inductance L of the wires is less than the other kinds of packages.
Although the MLCP shown in FIG. 1 is the best package known at present, the MLCP still has the disadvantage that the wires must become long, increasing the inductance L, if the number of wirings is increased above a certain amount.
In a package having a plurality of stacked ceramic plates each having metallized wirings formed thereon, vertical signal lines must generally also be employed so that the amount of cross-coupling between signal lines is increased. As a result, mixing or interference between signals is apt to be caused.
Further, there is another difficulty in that, since the signal lines and the power source lines formed on the vertically arranged ceramic plates are connected to each other via through holes, the characteristic impedance .sqroot.L/C cannot be made constant.
Moreover, this package fails to provide any terminating resistance equal to the characteristic impedance. Therefore, when an input signal is applied to a signal line, reflection of the signal occurs inevitably at the end of the metallized wiring.
It is difficult to provide, for example, chip resistors of 50 .OMEGA., when the number of input signals is increased. Although suitable for use in hybrid ICs or the like, such resistor chips are too large to be mounted on the metallized wirings of the package. Further, even if such a chip resistor were mounted on the metallized wiring, it would be difficult to attach the chip resistor to the end of the metallized wiring, that is, at the connection point between the metallized wiring and the wire.
The conventional IC package has served only as a so-called package, and has never been provided with any built-in resistors.
Further, although a MLCP has a good cooling property in comparison with a plastic package, in the case where a semiconductor chip mounting portion of the MLCP is made of alumina, the thermal resistance is 40 to 50.degree. C./W because the thermal conductivity of alumina is about 0.05 cal/cm sec .degree. C. Accordingly, the MLCP is unsatisfactory in view of its cooling property as a package for an ultra-high-speed IC having a large power dissipation. Further, if a terminating resistor, which is a heat generating element, is formed in the IC package, it becomes a more serious problem as to how to improve the cooling property in order to maintain the reliability of the semiconductor device and in order to minimize the limitations imposed by the TCR (resistance temperature coefficient) of the resistor material.